Editor's note: CNN's Jeanne Meserve and producer Mike M. Ahlers recently went along with a Transportation Security Administration official on an undercover test of airport screeners. Here is what they witnessed. CNN agreed not to disclose the name of the tester.

A mock bomb -- as slim as a wallet -- gets placed in a back support of a TSA tester going undercover as a passenger.

TAMPA, Florida (CNN) -- Jason -- that's the name CNN was asked to call him -- slides a simulated explosive into an elastic back support. The mock bomb is as slim as a wallet; its fuse, the size of a cigarette. He wraps the support around his torso, and the bomb fits comfortably into the small of his back.

It's hard to tell he's concealing anything; harder still when he dons a black T-shirt and a maroon golf shirt.

Then, with CNN's cameras in tow, Jason heads to Tampa International Airport, where he'll try to sneak the fake explosive past security screeners.

As jobs go, this one comes with its own unique set of satisfactions and tribulations. Jason wants to succeed at his task -- and he wants to fail. Success is a measure of his stealth, hewn by 40 years in law enforcement. But failure is satisfying too, because it means airport screeners are growing more adept at detecting threats.

So Jason -- looking every bit the middle-aged man on an uneventful trip to anywhere -- shows a boarding pass and an ID to a TSA document checker, and he is directed to a checkpoint where, unbeknown to the security officer on site, the real test begins.

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He gets through, which in real life would mean a terrorist was headed toward a plane with a bomb.

To be clear, the TSA allowed CNN to see and record this test, and the agency is not concerned with CNN showing it. The TSA says techniques such as the one used in Tampa are known to terrorists and openly discussed on known terror Web sites.

Even before the September 11, 2001, terror attacks, government agencies deployed "red teams" such as this one to look for holes in airport security. The tests have resulted in a torrent of reports criticizing the government for failing to staff, train, manage and equip properly the screener work force, which numbers 43,000.

While test results are classified and rarely leak out, those that have been disclosed typically don't inspire confidence. In tests conducted in 2006 and disclosed to USA Today last year, investigators successfully smuggled 75 percent of fake bombs through checkpoints at Los Angeles International Airport, 60 percent through Chicago's O'Hare International Airport and 20 percent at San Francisco International Airport.

The TSA has disputed some test methodologies and test results. But instead of running from tests, the agency has embraced the idea that testing has a value that goes beyond measuring the performance of individual screeners.

Tests,the TSA says, can show systemwide security vulnerabilities. When used frequently -- as was the case with San Francisco in 2006 -- they can heighten screener awareness. Tests can show areas that need increased attention. And tests can be used to determine whether terrorist plots uncovered by intelligence agencies or being discussed on terrorist Web sites are lunatic rants, or are plausible.

As a result, the TSA says it believes its work force is the most tested in the federal government, with checkpoint drills of various sophistication occurring in every checkpoint at every airport daily.

Almost an hour before Jason approached the checkpoint, a fellow red team member had gone through the checkpoint. It's this member's job to make sure the test is conducted safely.

Five minutes before the test begins, he uses a cell phone to call Tampa's federal security director, the airport's top security official.

"Sir, the reason we're calling today is to tell you that we will be conducting covert testing at your airport," the red team leader says. "But I would ask that you not speak to anyone on your staff to alert them of this test."

The message is clear: Don't tell anyone. Testers say they will scratch a test if they believe anyone has been alerted. The TSA was embarrassed several years ago when word of one test leaked out, and an internal auditor is investigating other possible leaks.

But in Tampa, everything goes smoothly as Jason steps through the metal detector portal. The detector alarm goes off, as Jason expects it to, not because of the nonmetallic device strapped to his back but due to his metal knee.

It's the perfect tool for ensuring he gets to "secondary," where more extensive searches are conducted.

Soon Jason is in a posture familiar to air travelers. He is standing, legs apart, with his arms extended. A screener "wands" him with a hand-held metal detector, and it beeps as it passes his metal knee, his necklace and the rivets on his bluejeans.

The screener then pats him down, running latex-gloved hands over Jason's legs, arms and torso. And he pats down Jason's back, including the lower part where the device is concealed.

But Jason explains away the back support. He tells the screener that he has a bum back in addition to having a metal knee.

With the patdown over, the screener releases Jason. He picks up his belongings and walks freely into the airport, the fake bomb still fastened to his back.

TSA officials say the Tampa test demonstrates the type of systemic vulnerability that the agency is working to expose and address.

Screeners have cultural sensitivities toward travelers' handicaps, and they are sometimes hesitant to perform intrusive searches, officials said. Terrorists could exploit that reluctance, they said.

The TSA screener could have used other relatively unobtrusive means to check Jason's back brace. But he didn't.

After leaving the screening checkpoint, Jason returns with other members of his red team and informs the screenerhe has failed a test. A fake bomb has just entered their airport.

The screener appears devastated.

The reaction is common, says Jason, adding that notifying screeners of failed tests can be the toughest part of his job.

On occasions, he says, testers have appeared indifferent. In those rare instances, Jason says, he gets "nasty," stressing the importance of the tests. The stakes are too high to tolerate indifference.

Regardless of their reactions, screeners who fail to detect contraband are "pulled off the line" and retrained before being allowed back.

The test CNN witnessed was conducted by the TSA's Office of Inspection, which the agency callsthe most sophisticated of its covert tests. But there are others.

For starters, every TSA X-ray machine has a Threat Image Projection system, which digitally inserts images of guns, knives and bombs into the X-rays of luggage, to keep screeners alert. This system library contains "tens of thousands" of images, said TSA spokesman Christopher White.

If screeners observe a suspicious object, they can check with the simple click of a computer mouse. If they detect a threat object, the computer congratulates them. Successes and failures are recorded for use in a screener's performance evaluation and are factors in determining pay.

Some 69,929 threat image tests are conducted on an average day, or more than 25 million tests per year. An array of other tests also are conducted to assess screeners, including the red team ones.

The TSA declines to give test results, which are classified. But it says the agency is getting better at finding bomb parts. And test scores won't demonstrate that, it says, because as success rates improve, tests are made more difficult.

"We're designing our tests not so much to indicate or to show or highlight performance," says Dave Holmes, who runs the Office of Inspection, "but we're highlighting where the vulnerabilities exist."

The elaborate test at the Tampa airport, Holmes says, is not to identify individuals performing below par. It's intended to provide data that, together with other information, will reveal the whole system's performance.

Back at the Tampa checkpoint, a member of Jason's red team is holding court with a group of screeners, including the one who missed the fake bomb.

"Today ... was a scrimmage," the red team leader says. "Every day, every time a passenger is coming through -- that is game day." E-mail to a friend